Various industrial production processes of acetic acid have been known. Among others, an industrially excellent process includes a process which comprises continuously allowing methanol to react with carbon monoxide with the use of a metal catalyst (such as a rhodium catalyst), an ionic iodide (e.g., lithium iodide), and methyl iodide in the presence of water in a carbonylation reactor to give acetic acid. In this process, usually, acetic acid is produced as follows: a reaction mixture of methanol and carbon monoxide, which contains acetic acid, is subjected to a distillation (a flash distillation) in a flasher (a flash evaporator), and a component vaporized by the distillation is subjected to a further distillation to separate (further purify) a component containing acetic acid.
The reaction mixture contains hydrogen iodide in addition to product acetic acid, ionic iodide, and methyl iodide. An increased concentration of hydrogen iodide inside the carbonylation reactor may precipitate the corrosion of the carbonylation reactor. Moreover, when the reaction mixture containing hydrogen iodide is subjected to the flasher or a further distillation column for separating acetic acid, or when a residue (liquid residue or bottom fraction) after the separation of the vaporized component is recycled to the reactor, the reaction system may be adversely affected, and additionally the corrosion of peripheral device(s) may be precipitated.
Therefore, in the production process of acetic acid, it is preferable that rising of the concentration of hydrogen iodide in the carbonylation reactor (or the reaction mixture) be prevented. Although a technique for inhibiting condensation of hydrogen iodide in a distillation column such as plate column, packed column has been already known, a technique closely focused on hydrogen iodide in a carbonylation reactor is not known.
For example, Japanese Patent Application Laid-Open No. 2006-160645 (JP-2006-160645A, Patent Document 1) discloses a process for distilling a mixture containing hydrogen iodide and water, which comprises distilling the mixture having a water content of not more than 5% by weight in a distillation system to prevent condensation of hydrogen iodide in the distillation system. In Examples of this document, a process solution (specifically, a volatile component separated by a flash distillation of a reaction mixture) free from an ionic iodide (such as lithium iodide) is examined for the effect of the water concentration on the hydrogen iodide condensation. Incidentally, the document discloses a wide range of compositions (or formulations) of the mixture (for example, the concentration of water is about 0.1 to 14% by weight, the proportion of a carbonylation catalyst is about 50 to 5000 ppm, the iodide salt content is about 0.1 to 40% by weight, the concentration of an alkyl iodide is about 1 to 25% by weight, the concentration of a carboxylate ester is about 0.1 to 20% by weight). However, the document is silent on any relationship between the composition of these components and hydrogen iodide in the carbonylation reactor.
As described above, the purpose of the conventional art is to condense hydrogen iodide in distillation, and the decrease of hydrogen iodide in a carbonylation reactor has not been examined.